5 Star Movement is poised to be the country’s largest party after Sunday’s national elections

ROME—Last October, Beppe Grillo headlined a protest by the 5 Star Movement, the populist political party he founded. Standing outside the Pantheon he denounced a new electoral law that will limit direct voting for parliamentary candidates and help protect incumbents in national elections on Sunday.

Mr. Grillo, who became famous as a comedian, brandished a white blindfold. “We are protesting blindfolded because this is how they want us—unable to see,” the 69-year-old told a cheering crowd. “They only pass rules that protect...

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